This invention relates generally to the field of measuring phase differences between electrical signals. The apparatus and method of this invention is particularly useful where phase differences are to be measured between relatively high frequency signals (e.g., up to 70 MHz or more) and/or where relatively small phase differences are to be measured.
As those in the art will appreciate, there are a wide variety of useful applications for phase measurement circuits/methods. Although this invention is of general applicability, the exemplary embodiment of this invention finds especially useful application for interferometer measurements made in plasma fusion devices (e.g., for measuring the line integral of electron density in the plasma). Such interferometers typically use very high intermediate frequencies (e.g., on the order of 10 to 70 MHz) and therefore the phase comparison circuitry should be a high speed circuit with a linear transfer characteristic so as to accurately differentiate between small fractions of interference fringes.
If the relative phase difference between two signals is to be measured with high resolution, then it is necessary that the system exhibit substantial linearity such that changes in relative phase produce a directly proportional and reproducible change in the overall output signal from the phase comparator. If non-linearity is present in the phase comparator, then the exact amount of phase change responsible for producing a given output signal is necessarily less accurately known so that less overall accuracy in the phase measurement is achieved.
Typical prior art circuits used in such interferometer applications unfortunately generally produce non-linear overall transfer functions at higher frequencies. Unfortunately, typical prior art phase comparator circuits used in the past for interferometry fail to resolve small phase differences, or are exceptionally non-linear. At least in part, such non-linearities are enhanced when only small phase differences are involved because the response/rise time of the pulses being output by the comparator have sloped, rounded or otherwise distorted leading/trailing edges (with respect to the theoretically perfect square waveshape). Since, in typical phase comparators, the output signal represents an integrated average of such pulses, even small distortions in the pulse shapes of relatively small pulses necessarily produces inaccuracy in the final averaged output signal.
Now, however, I have discovered a novel form of dual channel comparator method/apparatus which provides a substantial improvement when signals of relatively high frequency are to be compared and/or when signals having relatively small phase differences are to be compared.